1. Field of the Invention
This invention relates to a process for selectively forming rare earth carbonates from a mixture of rare earth oxides or hydroxides. In particular, oxides or hydroxides of lanthanum, neodymium, samarium, europium, gadolinium, dysprosium, holmium, promethium, thulium, and lutetium are found to be rapidly converted with high yield, to their respective carbonates using the process of the invention. This technique finds use in facilitating the extraction of these materials from rare earth containing mineral ores by providing a scheme for separating these particular rare earths from other rare earth and rare earth-like materials which do not react to form carbonates.
2. Description of the Prior Art
The rare earths, also known as the lanthanides or as lanthanons, and meaning here those elements having atomic numbers 57 to 71, are substances finding utility in high strength alloys, permanent magnets, petroleum refining, catalysis, phosphors and more recently, in high temperature superconductor materials. As demand for these materials grows, the availability of the rare earths and rare earth containing compounds becomes an ever increasing concern. While proven rare earth reserves are believed sufficient to meet current and expected demand, the methods of extracting the rare earths from source mineral ores such as monazite, bastnasite and xenotime, as well as processes for separating and isolating the various lanthanons from each other are beset with difficulties due to the extreme chemical and physical similarity occuring within the lanthanide family.
Ore extraction processes removing rare earths and rare earth containing compounds from an ore matrix are known which involve the initial formation of rare earth oxides or hydroxides by the use of heat, or by reacting the crushed ore with suitable solvents. The rare earth oxides or hydroxides thus formed are then further separated into various fractions of rare earth containing material. Individual rare earth compounds are eventually isolated by solvent extraction or ion exchange techniques, which have, by and large, replaced older separation methods such as fractional crystallization.
It has been these isolation and separation steps of rare earth compounds which have proven to be the most troublesome. The electronic configuration underlying the lanthanide family permits only small changes in atomic and ionic radii from one rare earth element to the next and nearly all the lanthanides are trivalent, the predominant oxidation state being +3. Accordingly, this physical and chemical proximity has required the use of separation and isolation techniques that are time consuming, expensive, and which quite often result in less than optimal yields.
To facilitate the separation of the rare earth compounds into various fractions, and to make easier the later isolation of them as from among each other, these rare earth oxides or hydroxides are often converted into rare earth carbonates. Carbonates are useful in that certain rare earths in the form of carbonates, have either more or less relative solubility in certain media. Carbonates are also readily converted into other compounds and are more easily stored.
The formation of lanthanide carbonates in order to facilitate processing is currently accomplished by one of several known methods, each of which has associated drawbacks. First, lanthanide carbonates may be precipitated from solutions which contain lanthanide ions by the addition of alkali carbonates or bicarbonates. However, in the presence of alkali metal ions, double salts, which may be represented as M.sub.2 (CO.sub.3).multidot.K.sub.2 CO.sub.3 nH.sub.2 O, where M denotes a rare earth element, are often formed. This is undesirable if pure lanthanide carbonates that are free of extraneous cations are required.
A somewhat more convenient method for the preparation of lanthanide carbonates is the hydrolysis of lanthanide trichloracetates in a homogenous phase reaction which can be depicted as: EQU 2M(Cl.sub.3 CCOO).sub.3 +(x+3)H.sub.2 O.fwdarw.M.sub.2 (CO.sub.3).sub.3 .multidot.xH.sub.2 O+3CO .sub.2 +6CHCl.sub.3
where again, M denotes the particular rare earth element involved. By the use of this method, as well as several of its known variations, all the lanthanide carbonates, M.sub.2 (CO.sub.3)3.multidot.xH.sub.2 O, have been synthesized, where x=8 for the light lanthanides of low atomic number (typically elements having atomic numbers 57 to 64) and 2&lt;x3 for the heavy high atomic number elements (typically elements having atomic numbers 65 to 71). The homogeneous phase hydrolysis however has several important drawbacks--namely, the lanthanide is not completely precipitated as the carbonate and the composition of the precipitate is dependent upon the conditions under which the precipitation reaction is carried out.
Lastly, lanthanide carbonates have been prepared by passing carbon dioxide through a suspension of lanthanide oxides, or hydroxides, in water. For example, U.S. Pat. Nos. 3,092,449 and 3,492,084 describe the formation of rare earth carbonates by bubbling carbon dioxide gas at temperatures and pressures which are at, or are only slightly above ambient conditions, through such a suspension. However, the conversion of the oxide, or hydroxide, into the carbonate is extremely slow and the reactions have been found to occur over a period of many hours to several days. Further, in many instances the yield of carbonates is low; the final reaction product being a mixture of the oxide or hydroxide and the carbonate. These shortcomings are overcome in the present invention where carbonate formation occurs with carbon dioxide that is in a supercritical state; that is at a temperature and pressure above 31.degree. C. and 72.9 atmospheres, respectively.
With respect to supercritical fluids, they are, as such, known primarily for their ability to act as extractants. In this regard, their transport properties are found to be between those of a gas and a liquid. Indeed, they are commonly thought of and characterized as either dense gases or, alternatively, as superheated liquids. For a fluid above its critical point, slight changes in temperature and pressure cause extremely large changes in density and thus dissolving power, which of course affects the ability to extract solute. By returning the fluid to a subcritical state, the extracted solute is removed in what has been characterized as a form of retrograde condensation. Supercritical carbon dioxide, the fluid of the present invention, has been widely used in this extractive capacity by the food industry to extractively remove, for example, caffeine from coffee, or acids from hops. Inorganic substances, such as those found in minerals, have also been extracted, as described in U.S. Pat. No. 4,457,812, by supercritical solvents which normally contain chloride. These solvents act to "take-up" and remove the various inorganic portions from the mineral.
Of more functional relevance to the present invention is the lesser-known ability of supercritical fluids to act as reactants, rather than as extractants. With regard to organic chemicals, oxidation of organics through the use of supercritical water is shown by U.S. Pat. No. 4,543,190. In the field of inorganics, which is more germane to the present invention, the preparation of potassium and rubidium amido-metallates of europium, yttrium and ytterbium, K.sub.3 M(NH.sub.2).sub.6 and Rb.sub.3 M(NH.sub.2).sub.6, has been described in "Uber Kalium-Und Rubidiumamidometallate Des Europiums, Yttriums Und Ytterbiums, K.sub.3 M(NH.sub.2).sub.6 Und Rb.sub.3 M(NH.sub.2).sub.6 ", Journal of the Less-Common Metals, 85 (1982) 97-110. These compounds were prepared in well-crystallized form by the reaction of the corresponding metals in supercritical ammonia acting as reactant and solvent. Supercritical carbon dioxide, acting as a reactant as in the present invention, has not been described in the art.